Impact ionisation in GaAs planar-doped barrier structures grown by molecular layer epitaxy

Abstract

The I–V characteristics and substrate current of GaAs planar-doped barrier (PDB) n+-i-p+-i-n+ structures with various barrier heights (0.5–0.8 V) grown by molecular layer epitaxy (MLE) have been measured at temperatures ranging from 77 to 296 K. The measurement was carried out at the constant source–drain distance of 1000 Å. It is found that an abrupt increase in the drain current occurs, in the high barrier (>0.73 V) PDB structures at a threshold bias voltage VTH. The value of VTH decreases from 3.1 to 2.7 V with increasing temperature from 77 to 296 K, and the coefficient is –1.8 mV/K. The ratio of VTH and GaAs bandgap Eg is obtained to be eVTH/Eg ≈ 2. The mechanism of the abrupt increase in the drain current is attributed to the effects of ballistic electron transport and the lowering of the potential barrier by the accumulation of impact-ionisation-generated holes at the p+ plane. Furthermore, this mechanism is confirmed by the results of photocurrent measurements, in which holes are generated by incident light. The obtained results have important implications for the design of the barrier height in PDB structures and other devices having thin potential barriers.